"Corn steep liquor" is the name given to the aqueous liquid which is obtained when corn (maize) is digested with warm water so as to soften and swell the corn grain. At the same time, soluble material in the grain is extracted into the steepwater and this material is, given the appropriate conditions, readily fermentable.
In practice, the corn is held in a series of silos through which passes the steepwater. The content of solubles in the steepwater increases as it passes through the series, the water leaving the last silo having the highest content of dissolved matter. The conditions under which the steeping is conducted are conducive to a lactic fermentation taking place in which reducing sugars are converted to lactic acid. As the corn steepwater leaves the last silo, this lactic fermentation is already in progress.
The corn steep liquor being rich in nutrients finds a use as a feedstock for the organisms used in the industrial production of antibiotics, such as penicillin. For this purpose, the corn steepwater from the steeping is submitted to an evaporative process to remove some of the water and thereby concentrate the desired nutrient solids.
It is important that the corn steep liquor used by the fermentation industry has a constant quality. Factors by which the quality is judged are a low content of reducing sugars, a high lactic acid content and a light brown color. The presence of high levels of reducing sugars in the corn steepwater gives rise to the formation of toxic compounds during the evaporation and sterilization of the corn steepwater thereby reducing the yield from a subsequent fermentation.
The use of corn steep liquor as a penicillin nutrient is also unsatisfactory if the liquor has a too high free lysine content. The lysine content of corn steep liquor is diminished to some extent by the fermentation processes taking place in the water during the steeping process but the disappearance of free lysine is slow and the amount left in the corn steep liquor solids after evaporation is frequently too high for their use in penicillin production.
U.S. Pat. No. 4,359,528 describes a process for producing a corn steep liquor which is said to facilitate the lactic fermentation to a maximum and to produce a product having a high concentration of dry matter, an almost total absence of reducing sugars, an extremely regular composition and a very low content of free lysine, histidine, arginine, aspartic acid and tyrosine. The process described in the U.S. patent for achieving these aims comprises contacting corn grains placed in silos arranged in series with a steepwater containing about 0.75 to 3 grams sulphur dioxide per liter, which water is introduced successively into each of the silos in series such that the water traverses the silos one after the other, the volume of steepwater in cubic meters introduced per ton of commercial corn defining a cycling ratio. The temperature of the steepwater is maintained so that the temperature decreases progressively from silo to silo as the water passes through the silos from one to the next, from at the most about 58.degree. C. in the input silo to about 32.degree. C. at the outlet of the silo from which the corn steepwater is recovered. The cycling ratio is maintained between 0.8 and 1.2 cubic meter steepwater per ton of commercial corn and the soak time during which the corn grains and steepwater are in contact is kept between about 24 and 44 hours.
As will be clear from the preceding description, the process of the U.S. patent requires a sophisticated temperature control system and at the lower temperatures there is the possibility of yeast fermentation occurring with the production of alcohol from the sugars present. The disclosure of the U.S. patent does briefly summarize two other solutions which have been proposed to encourage lactic fermentation but dismisses both as not giving satisfactory results.
The first of these two other solutions is said to consist of a complementary incubation in which the steepwater with 6 to 8% of dry matter is sent to a storage tank where it remains between 8 and 24 hours before being evaporated. This dwell time is said to permit more thorough exhaustion of the soluble materials by the lactic bacteria. The batch process is, however, comparatively slow and even more than 24 hours is often required to complete the process. Time periods such as this mean that a corn steeping plant has to be provided with relatively large incubators in which to conduct the process, hence, increasing the capital cost.
The second other solution is said to involve the seeding of the steepwater by means of lactic bacteria which would have the effect of reducing soaking time, the temperature of the soaking being comprised between 45.degree. C. and 50.degree. C. and the water cycling ratio used being 1.4 to 1.8 cubic meters of water per ton of commercial corn.
The process of the present invention does not involve changes to the steeping process itself but comprises a post-treatment of the corn steepwater after it has contacted the corn. It is notable for requiring a relatively short period of time for the treatment, by being operated continuously and by producing a product with an improved color and a very low content of reducing sugars. In addition, when desired, the process may be operated so as to produce a product with a low content of free lysine.